Conventional keyboards such as those disclosed in Taiwan Patent Nos. 445471, I220213 and M346861 generally include a keycap, a circuit board triggered by the keycap to generate a command signal, a baseboard located at one side of the circuit board and a link movement mechanism with two ends to bridge the keycap and the baseboard. The keycap is depressible to move towards the circuit board. The link movement mechanism is compressed downwards to press and set on a circuit switch on the circuit board to generate a corresponding control signal. If the keycap is not depressed to move towards the circuit board the link movement mechanism provides a return elastic force to push the keycap in normal conditions to move away from the circuit board at a selected elevation. While the aforesaid conventional keyboard structure can achieve the object of command input, it is constrained by the height of the link movement mechanism that makes the keyboard thicker. In view of the prevailing trend on the requirement of computer design that makes thin and lighter weight of products the main appeal, especially notebook computers, the thickness ratio of keyboard module is the main factor affecting the total thickness of the notebook computers. Hence all producers have devoted a great deal of efforts trying to make the keyboard structure thinner to reduce the total thickness.
In order to overcome the aforesaid problem, some producers have proposed another keyboard design, such as Taiwan Patent No. M434979 which provides a V-shaped structure consisting of a first support member and a second support member to solve the thickness problem resulting from the X-shaped structure of the conventional link movement mechanism; or Taiwan Patent No. M419973 which has an extensible lift structure located between the keycap and the circuit board to replace the conventional link movement mechanism to facilitate miniaturization; or Taiwan Patent No. M426075 which has a first support member and a second support member around the keycap to replace the conventional link movement mechanism to reduce total thickness of the keyboard. Although the aforesaid techniques can reduce the total thickness of the keyboard, they merely implement the link movement mechanism in other forms, thus the keyboard structure still has a certain thickness. Moreover, the structures they provided are difficult to make and assemble.